Not sure if it is even possible to fix this so it adds the correct eggs automatically, but building support for The Twilight Forest, Alex's mobs, & Divine RPG to name a few would be better than chicken spawn eggs
this was actually expected to happen and I found out why it didn't work. Fixed that in the latest release and so it should now work most if not all of the modded spawn eggs.
Not sure if it is even possible to fix this so it adds the correct eggs automatically, but building support for The Twilight Forest, Alex's mobs, & Divine RPG to name a few would be better than chicken spawn eggs